The present invention relates to electrical connections for superconducting devices, and more particularly to methods for soldering superconductive materials and to the bond created thereby.
Niobium is a widely-used material for superconductor thin-film device fabrication due to its high transition temperature, ruggedness and ease of deposition. However, one drawback of niobium is the difficulty in forming superconducting connections because of a hard oxide layer that forms on its surface.
Methods of making superconducting electrical connections include mechanical pressure contacts, spot welding, ultrasonic wire bonding and soldering. Pressure contacts require relatively bulky apparatus and are not reliable. Spot welding can be used only with electrically conductive substrates and is not applicable to use with thin-film devices. Ultrasonic wire bonding requires relatively sophisticated techniques. Thus soldering is a desirable bonding method, provided that the solder is superconductive.
Conventional lead-tin solder is superconductive at low temperatures, but does not adhere well to niobium because of the oxide surface layer. Niobium oxidizes extremely fast making conventional techniques for cleaning oxidation from a surface prior to soldering unsuccessful because re-oxidization occurs before the surface can be coated with lead-tin solder. Soldering to niobium can be performed using lead-indium or lead-bismuth alloys as described in U.S. Pat. No. 3,346,351. These compounds adhere to the oxidized surface of niobium, but are soft and corrode easily. Adding tin to lead-indium alloy solder to improve corrosion resistance produces a solder which will not adhere to the oxidized surface of niobium and merely forms solder balls on that surface.